The present invention relates to the biomedical arts, and in particular to implantable electrodes, their manufacture and surgical endoscopic installation. The present invention finds particular application in conjunction with cuff electrodes which are self-biased to helically curl around and snugly engage a nerve trunk, and will be described with particular reference thereto. It is to be appreciated, however, that the invention is also applicable to other types of implanted biomedical devices for introducing, monitoring, or removing matter or energy by helical engagement with body tissue.
Electrical activation of the nervous system has been shown in recent years to offer great hope in restoring some degree of lost sensory and motor function in stroke victims and individuals with spinal cord lesions. Ways in which electrical activation of the nervous system can be utilized to restore a particular function include: (1) the use of surface electrodes to activate the nerves in the general region of interest; (2) the use of intramuscular electrodes, also to activate the nerves in a general region; and, (3) the use of nerve cuff electrodes placed around specific nerves of interest and used to activate them specifically and singularly. The third alternative offers advantages over the first two in that it requires the least amount of stimulating current and hence a minimal amount of charge injected into the tissue. In addition, it allows easy excitation of entire muscles rather than merely parts of muscles, a common situation for the first two categories. Because the use of nerve cuff electrodes requires delicate surgery, they are usually contemplated only when excitation of specific, isolated muscles is desired or the generation of unidirectional action potentials is required.
One prior art cuff electrode includes a cylinder of dielectric material defining a bore therethrough of sufficient diameter to receive the nerve trunk to be electrically stimulated. The cylinder has a longitudinal split or opening to facilitate spreading the cuff open in order to receive a nerve therein. After installation, the longitudinal split is sutured or otherwise held closed. Although suturing holds the cuff in place, an electric current path is defined through the split which permits current leakage. Two or three annular electrodes are positioned on the inner surface of the bore for use in applying the electrical stimuli. The electric stimuli may be used to generate propagating nerve impulses or may be used to block naturally occurring nerve pulses traveling along the nerve trunk, or the like.
Another earlier nerve cuff electrode described in U.S. Pat. No. 4,602,624 encircles a nerve trunk or other body tissue with at least one medication or electrical energy conductive member held against the tissue and a non-conductive sleeve extending to either side of the conductive member. This earlier cuff is cylindrical and includes a self-curling sheet of non-conductive material which is self-biased to curl into a tight overlapping cylindrical spiral or roll around the nerve trunk. At least one conductive member is disposed adjacent an edge of the self-curling sheet. To install the above-described cuff, the self-curling sheet is held flat with the conductive member adjacent the body tissue to receive the cuff. Thereafter, the self-curling sheet is permitted to curl into a tubular spiral or cylinder around the body tissue in an overlapping fashion. Accordingly, this cuff electrode can be difficult to implant where access completely around the nerve is limited. Also, this earlier cuff is essentially inflexible axially.
U.S. Pat. No. 4,590,946 to Loeb describes a surgically implantable electrode system which includes two or more electrically conductive elements embedded in a helically wound substrate made of insulative material. The preferred substrate is a bio-compatible polymer material such as silicone rubber, having sufficient stiffness to maintain its helical shape during the surgical manipulations necessary to position the electrode around a nerve bundle. The substrate is pre-shaped or molded defining its overall spiral contour and open space within the helix, and thus cannot be fabricated in a planar configuration. A separate membrane pouch is needed to insulate the electrode from the adjacent body tissue. This pouch greatly increases the bulk of the electrode and thus increases the potential for mechanically induced neural trauma. Also, the lead-in conductors must be anchored by a strain relief. This being the case, the Loeb electrode is difficult to manufacture, somewhat stiff axially and hard to implant.
The open helix of the Loeb electrode requires the additional insulating membrane pouch to exclude current flow without, or include flow within, the pouch. The cuff electrode of the present invention is manufactured in a closed helix form which does not require an external pouch to control current flow.
Because of its stiffness, the Loeb electrode requires a turning or "threading" of the electrode onto a nerve for implantation and thus necessitates an open surgical procedure. The cuff electrode of the present invention is "self turning" or "self threading" when used in conjunction with a specialized installation tool and therefore does not require an open surgical procedure for implantation.
Lastly, the Loeb electrode requires molding during its fabrication and thus cannot be manufactured in a planar configuration. The electrode of the present invention, however, is fabricated in a planar configuration and then formed into a three dimensional structure as a result of its inherent self-biasing. Planar fabrication is an advantage because it allows thin-film production techniques to be employed as described in our pending patent application Ser. No. 07/871,352.
The present invention contemplates a new and improved helical cuff electrode which is readily installed and removed with minimum surgery and without damaging the nerve trunk or other tissue. The present invention further contemplates a method of cuff manufacture and endoscopic installation tools and methods.